1. Field of the Invention
This invention relates to a process for electrophotographic image formation in which a magnetic brush from a magnetic one-component toner is used to develop a latent image.
2. Description of Prior Art
Commonly used one-component magnetic toners are of relatively low resistance. They have, for example, a resistivity in the range of approximately 10.sup.2 to 10.sup.6 ohm-cms. This low resistance is a consequence of the accretion of conductive carbon components at the surface of the toner particles, thereby facilitating charging of the toner induced by the charge image on the photoconductor (in contrast to triboelectric charging of a two-component toner).
If the developed charge image on the photoconductor itself is used as the copy in the direct process, the noted low resistance of the toner usually has no effect on the achievable image quality. If an indirect, so-called transfer process is used, however, to transfer the developed charge image onto another image carrier, then this low resistance is troublesome because it causes both lower acutance and lower image resolution.
To understood this effect, what happens in the transfer process shall be explained briefly. After a charge image on the photoconductor is developed by means of a relatively low-resistance one-component toner, the latter is situated on the photoconductor with a positive induced charge, for example. By applying a negative potential to a transfer roller, say, the one-component toner is conveyed onto a receiving sheet which lies between the toner image and the roller and is destined to record the image, e.g., a sheet of paper.
Since the paper sheet is not highly insulating and the one-component toner is of relatively low resistance, the latter, to the extent that it is situated on the paper's surface, can have its charge reversed, i.e., this one-component toner on the paper sheet now receives a negative charge and therefore begins to migrate in the opposite direction, i.e., back to the photoconductor. This process is repeated in time as long as the relevant toner particles are situated spatially in the region of influence of the transfer field. It should be noted that the described charge-reversal process occurs more quickly the lower the resistance of the one-component toner.
Since the back-and-forth migration of toner does not occur in a straight line even in a stationary system of electrodes, the duration of action, broad field zone and low-resistance one-component toner commonly used in practice result in an untolerable blurring and unacceptable loss of resolution in the transferred image.
By using a transfer roller subjected to a voltage and pressed against the back of the receiving sheet, it is possible, because of the contact pressure, to realize a shortest possible transfer distance and a temporally and spatially limited transfer field acting on a particular toner particle in the region of the transfer-roller's directrix most closely adjacent to the photoconductor. The previously mentioned impairment of image quality can be reduced somewhat by these measures, but the roller transfer process is affected by additional disadvantages.
Thus, the transfer field should be switched on just after the receiving sheet enters the transfer area and switched off just before the sheet exits from that area so as to be able to use the entire format area and avoid the danger of voltage puncture or sparkover onto the photoconductor. The photoconductor would be damaged by such a puncture or sparkover. Furthermore, the transfer roller itself can attract toner particles which subsequently smudge the back of the next copy. For that reason, it is necessary to provide a special cleaning device for the transfer roller. Moreover, due to the high contact pressure of the transfer roller required for obtaining usable image quality, the danger of mechanical damage to the photoconductor also cannot be excluded. Finally, there are considerable difficulties in changing the paper format for the receiving sheet. Because of the danger of voltage sparkover at the edge of the sheet, the transfer field would have to be laterally limitable if receiving sheets of smaller format were also to be usable, which would require quite a sizable engineering effort and cause considerable operating problems.
The noted difficulties with the roller transfer process thus represent considerable disadvantages.